Biochar and Bonechar As Amendments to Reduce Lead Availability in an Oxisol From Brazil.

Activated charcoal is a material with high adsorption capacity for heavy metals commonly used in water and effluent treatments. This material can be produced from various raw materials of vegetable (Biochar) and animal (Bonechar) origin and has attracted attention of different research areas specially by soil scientists due to its ability to fix carbon in the soils for very large periods of time.The chemical processes that affect the behavior and bioavailability of metals in soils are related to their removal from the liquid phase and sorption onto the solid phase.The availability of metals in the soil can be influenced by pH, organic matter content, clay content, presence of oxides (hydroxide) of Fe, Al and Mn, and the redox state of the soil. In the case of lead, with the advent of the use of metals by man and mining, there has been an increasing concentration of this element in the soil. Other Pb sources include impurities of fertilizers, sewage sludge, pesticides, industrial wastes (unless used in agriculture) and combustion of fossil fuels containing additives such as lead. This paper considers the hypothesis that Biochar and Bonechar might have similar capacity to reduce Pb content of an Oxisol (Ox).Therefore, increasing doses of Biochar (Bi) and Bonechar (Bo) were added to an Oxisol (IVP) in order to reduce the Pb availability. The treatments were: Ox and Ox + 2.5, 5.0, 7.5 and 10% (v/v) Bo or Bi. In such treatments were determined specific surface area, bulk density, Pb remaining retention, and successive Pb extractions with water, 1M KCl, and Mehlich-1 solutions. The Bi and Bo materials were characterized in their basic chemical and physical attributes. The Oxisol was also characterized in its basic chemistry and mineralogy attributes. Lead maximum adsorption capacity (PbMAC) was determined using the Langmuir model. The Biochar is composed of poorly crystalline materials with few crystalline phases such as carbon, silicon and aluminum silicates. The Bonechar shows predominance of crystalline material identified as hydroxyapatite and fluoroapatite. The Ox, Bi and Bo PbMAC were 3984, 43478 and 41667 mg Pb kg-1 material, respectively. The SSA was higher by adding Bi than Bo, but with different behaviors. The addition of Bi and Bo decreased the remaining concentration, but the pattern was different from both amendments, demonstrating the existence of different mechanisms of retention. The successive Pb extraction with different extractors showed differences in the Pb retention mechanisms. The Biochar adsorbs Pb by ion exchange while the Bonechar has greater ability to reduce the percentage of extractable Pb, probably associated to formation of inner sphere complexes.